1. Field of the Invention
The present invention relates to an automatic transmission for an automobile.
2. Description of the Prior Art
In the prior art, the automotive automatic transmission is equipped with a plurality of planetary gear sets and frictional engagement elements such as brakes or clutches connected with the individual components of the planetary gear sets. Thus, the automatic transmission realizes each of gear stages by engaging the individual frictional engagement elements selectively.
Moreover, the four-speed shift gear mechanism of plural a forward four-speed automatic transmission is equipped with a planetary gear unit which is formed by coupling a single planetary gear and a dual planetary gear. The two planetary gears of the planetary gear set have their sun gears integrated and their pinions sharing the carriers. The carriers supporting the aforementioned pinions and the pinion meshing with the ring gear of the dual planetary gear are also integrated.
Moreover, the input shaft extending from the output member of the torque converter is connected through a first clutch C1 to a connection member and through a second clutch C2 to a sun gear. A third clutch C3 and a one-way clutch F0 are interposed between the connection member and the ring gear of the single planetary gear, and a fourth clutch C0 is interposed between the aforementioned connection member and a larger ring gear.
On the other hand, the aforementioned sun gear can be retained by a first brake B1 which is a band brake, and a second brake B2 and a one-way clutch F1 are interposed between the larger ring gear and casing. Moreover, the carrier is connected to an output gear which is positioned generally at the central portion of the shift gear mechanism.
In the automatic transmission thus constructed: at a 1st speed, the first clutch C1 is engaged; at a 2nd speed, the first brake B1 is engaged in addition to the first clutch C1; at a 3rd speed, the first brake B1 is released, but the third clutch C3 and the fourth clutch C0 are engaged; and at a 4th speed, the third clutch C3 is released, but the first brake B1 is engaged.
At the 1st speed, moreover, the one-way clutch F1 and the one-way clutch F0 are engaged. At a shift from the 1st to 2nd speeds, however, the one-way clutch F1 runs over to prevent the shocks which might otherwise be caused by the change in the engagement.
At a shift from the 2nd to 3rd speeds, on the other hand, the shocks due to the engagement change are prevented by adjusting both the timing of feeding oil to the brake releasing hydraulic chamber of a hydraulic servo for engaging and releasing the first brake B1 and the timing of feeding the oil to a hydraulic servo for engaging and releasing the fourth clutch C0.
At a shift from the 3rd to 4th speeds, moreover, the 3rd speed state is maintained by the second one-way clutch F0 when the third clutch C3 is released, and the retention of the first brake B1 is delayed by the hydraulic control to prevent the shocks due to the engagement change.
In the automatic transmission thus constructed, however, at the shift from the 2nd to 3rd speeds, the shocks due to the engagement change are prevented by adjusting both the timing of feeding oil to the brake releasing hydraulic chamber of the hydraulic servo for engaging and releasing the first brake B1 and the timing of feeding the oil to the hydraulic servo for engaging and releasing the fourth clutch C0. As a result, the hydraulic control system is complicated, and still worse it is difficult to achieve the optimum shift feel for all running conditions and all vehicles.
At the shift from the 3rd to 4th speeds, moreover, the first brake B1, which has been released at the shift from the 2nd to 3rd speeds, has to be engaged again. By thus repeating the engagement and release, the band composing the first brake (or band brake) B1 is subjected to changing stress which decreases its durability.
Especially the 3rd and 4th speeds are very frequently used, and the aforementioned first brake B1 is repeatedly engaged and released at the shifts inbetween so that its durability compromised.
Thus, there has been provided (as disclosed in U.S. Pat. No. 4,086,827) an automatic transmission, to which is added a one-way clutch for eliminating the engagement and release of the brake at the shift from the 2nd to 3rd speeds and for preventing the reverse rotation of the sun gear and accordingly the shift shocks.
Incidentally, in the automatic transmission of this kind, a new brake has to be interposed between the aforementioned one-way clutch and the casing so that the axial and radial dimensions are enlarged.